The present invention generally relates to an apparatus for controlling wafer temperature in a plasma etcher and a method for using such apparatus and more particularly, relates to an apparatus for controlling wafer temperature in a plasma etcher during a plasma-on state which includes a temperature sensor for sensing the wafer temperature and a flow control valve for increasing or decreasing helium flow rate in a helium cooling loop and a method for using such apparatus.
In the fabrication of modem integrated circuit devices, one of the key requirements is the ability to construct plugs or interconnects in reduced dimensions such that they may be used in a multi-level metalization structure. The numerous processing steps involved require the formation of via holes for the plug or interconnect in a dimension of 0.5 xcexcm or less for use in high-density logic devices. For instance, in forming tungsten plugs by a chemical vapor deposition method, via holes in such small dimensions must be formed by etching through layers of oxide and spin-on-glass materials at a high etch rate. A high-density plasma etching process. utilizing a fluorine chemistry is frequently used in the via formation process.
The via hole formation process can be enhanced by improving the etch directionality by a mechanism known as sidewall passivation to improve the anisotropy of the etching process. By utilizing a suitable etchant gas and reactor parameters, an etch-inhibiting film, normally of a polymeric nature, can be formed on vertical sidewalls. The etch-inhibiting film or the polymeric film slows down or completely stops any possible lateral etching of horizontal surfaces in the via hole. For instance, when a fluorine-containing etchant gas such as CFH3 is used, a fluorine-type polymeric film is formed on the sidewalls. Many photoresist materials may also contribute to the formation of a polymeric film on the sidewalls. After the sidewall is coated with a polymeric film, it is protected by the inhibitor film to preserve the line width or via hole diameter control.
In a modem etch chamber, an electrostatic wafer holding device, i.e., an electrostatic chuck or commonly known as an E-chuck, is frequently used in which the chuck electrostatically attracts and holds a wafer that is positioned on top. The E-chuck holding method is highly desirable in the vacuun handling and processing of wafers. In contrast to a conventional method of holding wafers by mechanical clamping means where only slow movement is allowed during wafer handling, an E-chuck device can hold and move wafers with a force equivalent to several tens of Torr pressure.
In an etch chamber equipped with a plasma generating device and an electrostatic chuck for holding a wafer, a shadow ring is utilized as a seal around the peripheral edge of the wafer. The shadow ring, sometimes known as a focus ring, is utilized for achieving a more uniform plasma distribution over the entire surface of the wafer and to help restrict the distribution of the plasma cloud to stay only on the wafer surface area. The uniform distribution function is further enhanced by a RF bias voltage applied on the wafer during a plasma etching process. Another function served by the shadow ring is sealing at the wafer level the upper compartment of the etch chamber which contains the plasma from the lower compartment of the etch chamber which contains various mechanical components for controlling the E-chuck. This is an important function since it prevents the plasma from attacking the hardware components contained in the lower compartment of the etch chamber. In order to survive the high temperature and the hostile environment, the shadow ring is frequently constructed of a ceramic material such as quartz.
A typical inductively coupled plasma etch chamber 10 is shown in FIG. 1. In the etch chamber 10, which is similar to a Lam TCP etcher made by the Lam Research Corp., the plasma source is a transformer-coupled plasma source which generates high-density, low-pressure plasma 12 which is decoupled from the wafer 14. The plasma source allows independent control of ion flux and ion energy. Plasma 12 is generated by a flat spiral coil 16, an inductive coil, which is separated from the plasma by a dielectric plate 18, or a dielectric window on the top of the reactor chamber 20. The wafer 14 is positioned several skin depths away from the coil 16 so that it is not affected by the electromagnetic field generated by the coil 16. There is very little plasma density loss because plasma 12 is generated only a few mean free paths away from the wafer surface. The Lam TCP plasma etcher therefore enables a high-density plasma and high-etch rates to be achieved. In the plasma etcher 10, an inductive supply 22 and a bias supply 24 are used to generate the necessary plasma field. Multi-pole magnets 26 are used surrounding the plasma 12 generated. A wafer chuck 28 is used to hold the wafer 14 during the etching process. A ground 30 is provided to one end of the inductive coil 16.
In a typical inductively coupled RF plasma etcher 10 shown in FIG. 1, a source frequency of 13.56 MHZ and a substrate bias frequency of 13.56 MHZ are utilized. An ion density of approximately 0.5xcx9c2xc3x971012 cm3 at wafer, an electron temperature of 3.5xcx9c6 eV and a chamber pressure of 1xcx9c25 m Torr are achieved or used.
In the typical plasma etch chamber 10, a cooling means for the wafer backside is provided in an E-chuck for controlling the wafer temperature during the plasma processing. This is shown in FIG. 2 for the plasma etcher 40. In the conventional plasma etcher 40, E-chuck 42 is provided for supporting a wafer 44 thereon. E-chuck 42 can be constructed of either a metallic material or of a polymeric material. A plurality of ventilation apertures (not shown) are provided in the E-chuck surface such that a cooling gas can be supplied to the backside 46 of the wafer 44 during plasma processing. The plurality of ventilation apertures in the E-chuck 42 is connected in fluid communication with a cooling gas inlet conduit 38 for feeding a cooling gas into the apertures. The cooling gas inlet conduit 38 is in turn connected to a gas supply line 36, a flow control valve 34 and a cooling gas supply 32. The pressure in the cooling gas supply line 36 is monitored by a pressure sensing device 48 which in turn sends a signal 50 to a controller 52. The controller 52, after receiving signal 50 and comparing to a pre-stored value, sends signal 54 to the flow control valve 34 for opening or closing the valve and thus increasing or decreasing the cooling gas supply through the supply line 36, 38 into the E-chuck 42. The amount of the cooling gas that is supplied to the E-chuck 42 is further adjusted by a needle valve 56 and pumped away by a pump 58.
As shown in FIG. 2, the conventional method for controlling the E-chuck temperature and the wafer temperature is ineffective since there is no feedback control loop for achieving an efficient control of the cooling gas pressure that flown through the E-chuck 42. The temperature of the wafer 44 during plasma processing can not be detected and thus, the temperature can exceed a critical limit to cause a detrimental effect on the coating layers on the wafer. For instance, during a plasma etching process conducted on a dielectric layer, the wafer temperature can increase to such an extent that a photoresist layer coated on the wafer starts to flow during the plasma-on period. The lack of precise control on the wafer temperature in a plasma etcher therefore leads to severe processing difficulties and produces low yield of the wafer.
A test conducted and data obtained on an E-chuck equipped with conventional cooling apparatus is shown in Table 1.
As shown in Table 1, the wafer positioned on the conventional E-chuck rises to a significantly higher temperature during plasma-on that it is impossible to control the wafer temperature only by using the backside cooling gas. The backside cooling gas used can be any inert gas. This is the case when the heat-transfer medium flown between the wafer and the E-chuck is a helium gas, with the chamber pressure in the mini-Torr range and the backside helium pressure in the Torr range during the plasma-on state. When the backside helium cooling gas fails to effectively cool the wafer, a photoresist layer coated on the wafer may flow due to the excessive temperature during the plasma-on state.
Table 1 shows data obtained in two separate tests of poly and metal etch. Thermal dots are placed on the wafer to measure the maximum wafer temperature at specific locations during the plasma-on, or plasma-struck state. The results indicate that, for metal etching, a large temperature differential between the E-chuck and the wafer exists, i.e., as high as 33xc2x0 C. During the poly etch process, a smaller temperature difference of 10xc2x0 C. is observed. These data indicates that during the metal etching process, a temperature differential of 33xc2x0 C. must be accounted for, i.e., or must be controlled. When not controlled, the excessively high temperature affects the etch rate, the uniformity of etching, and the etch profile obtained. Moreover, when the wafer temperature is higher than 80xc2x0 C., a photoresist layer on the wafer may flow and thus causing the scrap of the entire wafer.
It is therefore an object of the present invention to provide an apparatus for controlling wafer temperature in a plasma etcher during a plasma-on state that does not have the drawbacks or shortcomings of the conventional control apparatus.
It is another object of the present invention to provide an apparatus for controlling wafer temperature in a plasma etcher during a plasma-on state that can be utilized in either a metal etching, a poly etching or a dielectric etching process.
It is a further object of the present invention to provide an apparatus for controlling wafer temperature in a plasma etcher during a plasma-on state which includes a temperature sensor for sensing the temperature of the wafer backside.
It is still another object of the present invention to provide an apparatus for controlling wafer temperature in a plasma etcher during a plasma-on state that includes a temperature sensor for sensing the wafer backside temperature and a flow control valve for increasing or decreasing the flow of cooling gas to the backside of the wafer.
It is yet another object of the present invention to provide an apparatus for controlling wafer temperature in a plasma etcher during a plasma-on state by utilizing a helium cooling loop that includes a first flow control valve, a first and second plurality of ventilation apertures and a second flow control valve.
It is another further object of the present invention to provide a method for controlling a wafer temperature in a plasma etcher during a plasma-on state by detecting a temperature of the wafer backside during plasma processing and then opening a flow control valve to increase a cooling gas flow to the backside of the wafer for enhanced cooling of the wafer.
It is yet another further object of the present invention to provide a device for cooling X a wafer during plasma etching that includes a first flow control valve, a second flow control valve, a temperature sensor for sensing wafer backside temperature, and a controller for opening the second flow control valve to increase a cooling gas flow when a temperature rise on the wafer backside is detected.
In accordance with the present invention, an apparatus for controlling wafer temperature in a plasma etcher during a plasma-on state and a method for using such apparatus are disclosed.
In a preferred embodiment, an apparatus for controlling wafer temperature in a plasma etcher during a plasma-on state can be provided which includes an E-chuck for holding a wafer thereon, a first and second plurality of ventilation apertures in the E-chuck for flowing a cooling gas into and out of a cavity formed between the E-chuck and the wafer, a gas inlet conduit for feeding a cooling gas to the cavity through the first plurality of ventilation apertures, a gas outlet conduit for exhausting the cooling gas from the cavity through the second plurality of ventilation apertures, a first flow control valve in the gas inlet conduit, a second flow control valve in the gas outlet conduit, a thermal sensor mounted in the E-chuck for sending a signal indicative of a temperature of the cooling gas in the cavity to a controller, and a controller for opening the second flow control valve and for increasing the cooling gas flow when a temperature rise in the cooling gas is detected.
In the apparatus for controlling wafer temperature in a plasma etcher, the apparatus may be used to control the temperature of the wafer during a plasma-on state. The apparatus may further include a pump in fluid communication with the second flow control valve for evacuating the cooling gas. The cooling gas used may be an inert gas, or may be helium. The first flow control valve, the first and second plurality of ventilation apertures and the second flow control valve form a helium cooling loop. The first and second flow control valves are mass flow controllers. The apparatus may further include a pressure gauge in the gas inlet conduit for sensing a gas pressure and sending a signal to the controller. The flow rates through the first and second flow control valves determine a cooling gas leak rate.
The present invention is further directed to a method for controlling a wafer temperature in a plasma etcher during a plasma-on state which can be carried out by the operating steps of first providing an E-chuck which has a first and second plurality of vent holes therethrough in fluid communication with a cavity formed between the E-chuck and a wafer positioned thereon, flowing a cooling gas into the first plurality of vent holes through a first flow control valve, flowing the cooling gas out of the second plurality of vent holes through a second flow control valve, detecting a temperature of the cooling gas in the cavity by a thermal sensor, and adjusting a cooling gas flow rate through the cavity by adjusting the second flow control valve to change the temperature of the cooling gas.
The method for controlling a wafer temperature in a plasma etcher during a plasma-on state may further include the step of controlling a temperature of the cooling gas by a controller. The method may further include the step of withdrawing the cooling gas from the second flow control valve by a pump. The method may further include the steps of switching on an RF power for the plasma source and opening the second flow control valve to cool the wafer. The method may further include the step of supplying a helium gas as the cooling gas, the step of positioning a wafer on the E-chuck before switching on the E-chuck, or the step of sensing a pressure in the first flow control valve by a pressure sensor and sending a signal to a controller.
The present invention is still further directed to a device for cooling a wafer during plasma etching which includes a first flow control valve for controlling a cooling gas flown into an E-chuck, a second flow control valve for controlling a cooling gas flown out of the E-chuck, a temperature sensor for sensing a temperature of the wafer being plasma etched, and a controller for opening the second flow control valve and for increasing the cooling gas flow when a temperature rise in the wafer is detected.
In the device for cooling a wafer during plasma etching, the cooling gas utilized may be a helium gas. The first and second flow control valves may be mass flow controllers. The device may further include a pump for evacuating the cooling gas from the E-chuck.